1. Field of the Invention
The present invention relates to a fluid compressor, and more particularly, to a compressor for compressing refrigerant gas in a refrigeration cycle, for example.
2. Description of the Related Art
An example of a fluid compressor is shown in U.S. Pat. No. 4,875,842 to applicants of the present application. This type of compressor comprises a closed casing, and a compressor unit and a motor unit both housed in the closed casing. The motor unit has a stator fixed to the inner surface of the casing, and a rotor arranged within the stator coaxially therewith.
The compressor unit has a cylinder fixed coaxially within the rotor and rotated integrally with the rotor. A piston is rotatably arranged within the cylinder such that the piston is eccentric in respect to the axis of the cylinder. A spiral groove is formed on the outer circumferential surface of the piston, extending from one end of the piston to the other end thereof. The pitches of the spiral groove are gradually reduced with distance from the suction side of the cylinder towards the discharge side thereof. A spiral blade having a suitable elasticity is fitted in the spiral groove.
The space defined between the cylinder and the piston is divided into a plurality of operating chambers by means of the aforementioned blade. The capacities of the operating chambers are gradually reduced with distance from the suction side of the cylinder towards the discharge side thereof. When the cylinder and the piston are rotated by the motor unit in synchronous with each other, refrigerant gas in the refrigeration cycle is introduced into the operating chambers through the suction end of the cylinder. The sucked gas is gradually compressed while being transferred to the discharge end of the cylinder. The highly pressurized gas is discharged from the discharge end of the cylinder into the closed casing.
The suction-side end portion of the cylinder i rotatably supported by a main bearing secured to the inner surface of the casing. The discharge-side end portion of the cylinder is engaged with a sub-bearing. The sub-bearing is arranged to be movable in the radial direction of the cylinder in relation to the casing. Namely, the cylinder is substantially cantilevered by the main bearing.
The rotor of the motor unit is secured on the middle of the cylinder in its axial direction. The center of gravity of the motor unit, especially the center of gravity of the rotor, is situated at the middle of the cylinder in its axial direction. Thus, the center of gravity of the rotor is located a long distance from that surface of the main bearing which is secured to the casing.
As has been stated above, in the compressor having the cylinder supported substantially at its one end, the rotational movement characteristics are greatly influenced by the state of the supported cylinder, rotor, etc. When the respective members are not precisely aligned, for instance, when a deviation appears between the axis of the rotor and that of the cylinder or between the axis of the rotor and that of the stator, the cylinder and the rotor may whirl, with respect to an intersection between the fixed surface of the main bearing and the center axis of the cylinder. In particular, when the center of gravity of the rotor is greatly separated from the secured surface of the main bearing, as in the case of the above-described compressor, such whirling of the cylinder and rotor is worsened. As a result, vibration and noise of the compressor increases, and the frictional loss at the sliding portion of the bearing is considerably increased, thereby lowing the operation efficiency of the compressor. Furthermore, the rotor and the stator may contact and damage each other.